I think I blew a power valve. Can I limp home? - Team Camaro Tech
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post #1 of 42 (permalink) Old Jan 28th, 11, 12:20 PM Thread Starter
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Exclamation I think I blew a power valve. Can I limp home?

Nice day today (62) so took the car to work. Got to work no problems. About an hour later I had to run to an off-site meeting and the car backfired first time ever (really LOUD) when I was starting it. Been having some issues with cold starts lately but this was the first GaBOOOOM ever. Car did start and I went to the meeting. Car started fine after the meeting but as I was pulling out from the parking lot, I got on it pretty good. The car died shortly before the 4-way stop as I was coming to a stop and I got a very strong whiff of gas. After a few cranks the car started back up and I went to lunch. Car started ok after lunch but it died again after I accelerated (respectfully) in order to make a green/yellow light. It died shortly after I let off the throttle. Pulled over and it started up after few cranks. Seems like the car will run as long as there is no hard acceleration.

My initial prognoses is that the backfire may have blown the power valve in my Holley 850, hence I am running way rich especially after acceleration where there is enough gas getting through to choke up the motor. I know that these carbs have a pressure relief valve just to accommodate a backfire situation but this backfire sounded like a cannon going off. Assuming I am right, do you guys think I can limp home? Drive slow, no hard accelerations what so ever, 15 mile drive, side streets...????

Matt

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post #2 of 42 (permalink) Old Jan 28th, 11, 12:57 PM
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Re: I think I blew a power valve. Can I limp home?

Usually a strong backfire thru the carb will fracture the PV. You can drive it home. Also, you can fatten up the carb by 4 or 5 jets bigger than what you have now and do away with the PV.

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post #3 of 42 (permalink) Old Jan 28th, 11, 01:48 PM
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Re: I think I blew a power valve. Can I limp home?

Ya know, I keep hearing about blown power valves, and suggestions that the power valve may be blown. But you go to the Holley website and it clearly says you can't blow a Holley power valve since 1992??? So, I'm just curious, is Holley full of it or are we?

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post #4 of 42 (permalink) Old Jan 28th, 11, 02:06 PM Thread Starter
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Re: I think I blew a power valve. Can I limp home?

Left work early to avoid traffic and made it. Almost stalled once and I smell like gas. Cooling the motor now, will be pulling the carb apart shortly. Yes, I have seen that claim on Holley's website that power valves never blow....

Matt

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post #5 of 42 (permalink) Old Jan 28th, 11, 03:45 PM
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Re: I think I blew a power valve. Can I limp home?

Be sure to change your oil too as you may have got fuel washed into the crank case past the rings.

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post #6 of 42 (permalink) Old Jan 28th, 11, 03:50 PM
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Re: I think I blew a power valve. Can I limp home?

You can buy a kit that is easy to install the prevents the power valve from blowing out. You just drill out your metering block and install a small ball bearing, that's it.

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post #7 of 42 (permalink) Old Jan 28th, 11, 05:38 PM Thread Starter
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Re: I think I blew a power valve. Can I limp home?

Well got it all back together...I changed the plugs too but they were actually pretty decent. Took the car for a spin and it seemed to run fine. It did not die after a couple pulls. BTW, asked the guy at the speed shop if he gets a lot of guys with blown power valves. He said ALL THE TIME! Yes, wouldn't be a bad idea to change the oil....I am due for a change anyway.... I will run her again tomorrow to make sure it's still all good.

Matt

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post #8 of 42 (permalink) Old Jan 28th, 11, 06:22 PM
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Re: I think I blew a power valve. Can I limp home?

So you bench tested the suspect blown power valve you replaced?
What was your findings?Ruptured diaphragm?

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post #9 of 42 (permalink) Old Jan 28th, 11, 06:29 PM
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Re: I think I blew a power valve. Can I limp home?

Quote:
Originally Posted by Eleanor's Nemesis View Post
Usually a strong backfire thru the carb will fracture the PV. You can drive it home. Also, you can fatten up the carb by 4 or 5 jets bigger than what you have now and do away with the PV.
I wouldn't do that on a street car. Running fat all the time is hard on cylinders.
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post #10 of 42 (permalink) Old Jan 28th, 11, 06:36 PM
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Re: I think I blew a power valve. Can I limp home?

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Originally Posted by Eleanor's Nemesis View Post
Also, you can fatten up the carb by 4 or 5 jets bigger than what you have now and do away with the PV.
If you are suggesting a secondary block off and an upjet by 8 numbers..I am ok with that.
If this is a street deal...he needs to retain the primary power valve for any semblance of driveability and mpg.

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post #11 of 42 (permalink) Old Jan 28th, 11, 07:11 PM
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Re: I think I blew a power valve. Can I limp home?

If it happens again, change the coil.... That may be the corporate to the backfire.

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post #12 of 42 (permalink) Old Jan 28th, 11, 07:27 PM
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Re: I think I blew a power valve. Can I limp home?

I say if it happens again, go EFI. You'll be glad you did if you ever try to take that thing up to Mt. Elbert or something. You'll just have to stop car shopping on craigslist for a month or two.

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post #13 of 42 (permalink) Old Jan 28th, 11, 08:01 PM
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Re: I think I blew a power valve. Can I limp home?

Quote:
BTW, asked the guy at the speed shop if he gets a lot of guys with blown power valves. He said ALL THE TIME! Yes,
And has he checked the "blown" PV...does he actually know how to?
Or is it ppl coming in with the old wives tales about back fire/blown PV...getting a new one any way, and simply by pulling the carb down, have dislologed the crap unknowingly and A55 -U -me d it was the PV.....?

Did you pull the air filter off, hold the rpms up, put your hand over the top, pull a huge vaccuum, pull your hand off just before it stalls, and repeat 3 or 4 times.

THE 1st side of the road old school trick to fixing a carb that has crap in it or flooding/ not working right....and 9 time out of 10 (more than that) it works, and keeps on working for the next few yrs .

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post #14 of 42 (permalink) Old Jan 28th, 11, 09:04 PM Thread Starter
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Re: I think I blew a power valve. Can I limp home?

Well.... The saga takes a turn, perhaps it wasn't the power valve after all. There was no noticeable damage visible on the power valve. I don't a have vacuum kit required to really test it properly. One other fact I have not mentioned up to this point. After the huge backfire I noticed that my idle rpms went up 200, just that 200. I used to idle in gear at around 800. All the sudden I am at 1000. Even after installing the new power valve, the rpms did NOT drop. OK, not sure why but let's back off on the main idle screw and see what happens. I turned it a good turn and half and rpms did NOT drop, nothing. Started looking much closer as to why the rpms would not back off with a good turn on the idle screw. Come to find out I noticed that my choke lever (my "choke" is not hooked up, and I don't have the choke butterfly on) was stuck in the middle position.

Below is what I believe is the zero choke lever position:



Below is what I believe is the full choke lever position:



Below is where I found my choke lever (red line). When in this position and looking at the other side of the carb, there is a "gap" between the main idle screw and the throttle lever hence the throttle does not retract all the way, hence the extra 200 rpms. Sure enough, pull the choke lever to the OFF position and the rpms drop as the throttle lever drops and rests on the main idle screw.



My carb understanding is very basic. I always through the choke was nothing but a rotating flap over the carb, when "on" resulting in less air to the carb hence car runs richer. Obviously there are other things in the formula as things do shift on the other side of the carb when in the choke mode, hence my idle going up. ....choke circuit...???

Could the pressure and rattle of the major backfire cause the choke lever to shift? Bigger question, if that's what it was, why would the car stall on me during the day and not when I took it for a test drive tonight before realizing that choke has shifted? Only thing I can think of is air temp. Today was a nice day, actually I think we hit 70. My car ran at 175 today verses 130 tonight. Could the hotter engine expose the choke-on/rich issue while the cold engine hide it?

Matt

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post #15 of 42 (permalink) Old Jan 28th, 11, 09:26 PM
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Re: I think I blew a power valve. Can I limp home?

I hate to tell you this. But you don't have a choke. The blade has been removed. i think you just had a stuck needle and seat. happens all the time on a holley. especially when they set up for a while. Keep a screw driver handy to tapp the bowles with
Larry

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